This invention relates to vehicle kinematic control systems of the kind that include an electric motor that is operable to alter the track of at least one rear wheel of a front wheel steer vehicle.
Most road going vehicles, in particular passenger cars, vans, lorries, trucks, buses and so on are conventionally steered by a driver operating a steering wheel to turn the front wheels of the vehicle. By turn we mean vary the track of the front wheels.
In addition, it is known to provide an additional amount of steering of the rear wheels of the vehicle in combination with the steering of the front wheels to improve the vehicle kinematics. Active kinematic control allows the track of both rear wheels of the vehicle to be regulated actively and independently for each wheel thus ensuring high steering precision. Thus, it may be used to improve for example agility and handling when parking, driving at low speeds and in city traffic. This is achieved by counter-steering the back wheels. At the same time, the turning circle of the vehicle is reduced. On the other hand, in-phase steering of the rear wheels may be used to stabilize the vehicle at high speeds, especially during evasive and lane change manoeuvres.
The benefits of the additional rear steering are especially noticeable in a high performance vehicle such as a sports car or supercar, where the stability of the vehicle during lane changes can be optimized at high speeds yet at the same time the manoeuvrability at low speeds can be optimized.
The turning of the rear wheels is achieved by varying the track of the rear wheels relative to the vehicle chassis, and this can be conveniently achieved by attaching a track rod of the rear wheel hub assembly to an end of an axially movable rack of a rack and pinion gearbox, the pinion of the gearbox being driven by an electric motor. A motor controller provides drive signals to the motor that are required to achieve the desired position of the rack and hence the track of the rear wheel. For convenience, the centre position of the motor may be defined as the position at which the rear wheels track dead ahead.
Both of the rear wheels may be turned together using a single motor, or there may be a motor provided for each of the rear wheels. Generally the vehicle will have a single pair of rear wheels, although more than one pair can be provided for a larger vehicle such as an over length truck.
A problem can occur when the motor fails with the track of the rear wheels in an off-centre position, i.e. both rear wheels not in line with the chassis main axis. This is a low risk failure because the steering angle possible from the rear wheels is generally much lower than the steering angle that can be applied to the front wheels, perhaps at most 3 degrees from the straight ahead, allowing the driver to compensate for the failure by adding a little more or a little less steering angle to the front wheels. Nevertheless, any fault could lead to an unusual sensation to the driver as the vehicle crabs along the road as if it is in a permanent state of oversteer or understeer. If the motor is moving rapidly and randomly it could also be distracting to the driver and lead to reduced confidence in the vehicle handling.